Light-emitting diodes (LED) offer many advantages over conventional lighting apparatus, such as long lifetime, high efficiency, and non-toxic materials. With the development of electronic technology, light-emitting diodes are finding ever wider applications. For example, in consumer applications, LED light bulbs are showing promise as replacements for conventional white light incandescent or fluorescent light bulbs. Further, more and more electronic devices adopt LCD as display, and LEDs are becoming increasingly popular as a backlight source.
Constant-current linear regulators are widely used as LED drivers because of their simplicity, low cost, stability, and suitability for PWM dimming. In order to provide a high power factor, a conventional LED controller uses linear regulators to control a string of LEDs. The controller can vary the number of LEDs to be turned on according to the magnitude of the input voltage. In other words, the controller turns on more LEDs in the string as the input voltage increases and reduces the number of turned-on LEDs as the voltage decreases. As a result, the total current flowing through the LED strings varies in a staircase manner that tracks the input voltage, thereby improving the power factor.
A disadvantage of these controllers is that the total LED current can vary with changes in supply voltage or operating temperature. For example, the total LED current is higher at a higher input voltage, and the LED current is lower at a lower input voltage. As a result, the brightness of the LED string can vary with the input voltage. These controllers can also be susceptible to LED forward voltage Vf variation due to device-to-device variations and temperature coefficient (TC). These conditions prevent the LED currents from having a desired steady average LED current leading to variations in the brightness of the LED lighting device.